I'm sorry but I just can't help myself. The Aden is a 30mm cannon that was never fitted to Tornados. The Mauser that is fitted to the GR4 is a 27mm cannon.
Relatively easy to Retro fit...................... just think of a number add lots of zeros and there you have it.
Should really add that if you wish your retro fit to work................ not correctly just work that requires lots more money.......................... there are shareholders and directors bonuses to think of.